High-Performance Boundary Control for NPC-DAB Converters: Multitrajectory Natural Switching Surface Derivation and Implementation

As the preferred topology for medium or high voltage applications, neutral-point-clamped dual-active-bridge (NPC-DAB) converters have recently received widespread attention. The main objectives of NPC-DAB converters include high steady-state efficiency, dc-bias elimination, fast dynamic characteristics, constant-power-load stability, and midpoint voltage balance (MVB). NPC-DAB converters may also experience various complicated operating conditions, such as startup, sudden reference voltage, and load change. Originating from the concept of natural switching surfaces (NSS), this article proposes a multitrajectory NSS boundary control (MNBC) to address these issues for the first time. First, the NSS mathematical models under various operating states of NPC-DAB converters were derived, laying a theoretical foundation for the proposed MNBC. With the proposed MNBC, elliptical and circular NSS will be used to flexibly adjust the dynamic trajectory of NPC-DAB converters in the geometric domain. Especially by switching the elliptical NSS, the charging and discharging states of capacitors in NPC-DAB converters can be changed without affecting the output waveform. This article provides a detailed analysis and derivation process of the proposed MNBC algorithm, as well as simulation and experimental evaluations. The results indicate that the algorithm performs well in terms of efficiency, dynamic response, and stability under various operating conditions.